This invention relates to circuit film strips and to methods for manufacturing such film strips.
The phrase "circuit film strip" is used herein with reference to a continuous strip of insulating film having indexing or sprocket holes extending along its side edges and having a repetitive pattern of conductors on one surface thereof. Circuit films of this type are being widely used in the electronics industry in the production of integrated circuit devices of the type comprising an integrated circuit chip to which a plurality of leads are bonded. The chip and portions of the leads which are adjacent to the chip may be encapsulated in a plastic body and the portions of the leads which extend externally of the plastic body are connected to circuitry on a printed circuit board or otherwise connected into electronic apparatus.
The IC chips are quite small and the bonding of the leads to the chips must be carried out with a high degree of precision. At the same time, the economic requirements of the electronics industry are such that all of the operations must be carried out at a low cost, preferably by methods involving a high degree of automation.
It is now common practice inthe art to form the very fine leads which are to be bonded to an IC chip on a film similar to a cinematic film having sprocket or indexing holes at spaced intervals along its side edges. The film can then be fed through assembly and other processing machines so that the chips can be bonded to the ends of the leads and intermediate portions of the lead scan be bonded to a lead frame (a metal frame having leads of relatively coarse size). Thereafter, the plastic material can be molded around the chip and the leads to produce the finished integrated circuit unit in condition for use in an electronic apparatus.
It can be appreciated that the circuit film must be precisely positioned in all of the processing and/or assembly machinery discussed above because of the extremely small size of the strip, the leads on the film and the IC chip and the achievement of such precise positioning has proved to be a vexatious problem. This problem arises from the fact that it is extremely difficult to produce sprocket holes in the film which are in precise registry with the circuit patterns and if the sprocket holes are not in such precise registry, the circuit pattern which is located in a work station in a processing machine will not be located in that work station with a sufficient degree of precision to permit successful completion of the operation which is being carried out.
In accordance with the principles of the instant invention, sprocket holes or index holes can be produced which are at precise spaced-apart intervals on the circuit film so that every time the film is indexed a distance equal to the spacing between two adjacent holes, the distance the film is actually fed will lie within extremely close dimensional tolerances. Furthermore, each circuit pattern on a film made in accordance with the invention is specifically associated or related, in a dimensional sense, with two or more adjacent sprocket holes so that each individual circuit pattern can be located in the operating zone of the machine through which the film is being fed with an extremely high degree of precision and within extremely close dimensional tolerances.
It is an object of the invention to provide an improved circuit film strip. A further object is to provide a circuit film strip of improved dimensional precision as regards the indexing or sprocket holes in the film. A further object is to provide a circuit film strip having circuit patterns thereon and having indexing holes in which each circuit pattern is specifically dimensionally associated with two or more of the holes. A still further object is to provide an improved method of manufacturing circuit film strip.